This application covers our invention relating generally to double acting simplex plunger pumps. Our invention specifically is applicable as a modification and improvement of the pump invention described and claimed in the patent application of James E. Cook filed concurrently with this application Ser. No. 07/766,331, and filed Sep. 27, 1991. The aforesaid improvements may generally be described as an improved precision aligning means and an improved structural or holding means for aligning and holding together the individual component parts of the pump.